This invention relates to the field of thermal insulation and, more particularly, to the field of purged multi-layer insulation systems.
The Space Shuttle promises to propel space travel and exploration to new reaches. For the first time in the short history of space exploration, a launch vehicle offers a large cargo bay for a wide variety of payloads and can, after orbiting the earth, return to the surface with or without its payload, land on a runway, be refitted with a new payload and be relaunched. The Shuttle is most certainly only the first of a long line of vehicles that will provide unparalleled opportunities for examining the stars, the earth and the universe. Other vehicles which offer even more capability than the Space Shuttle are on the drawing boards.
Before these opportunities can be provided, and the capabilities of these new vehicles realized, many technological limitations presently part of the spacecraft design art must be overcome. Spacecraft design has always demanded innovative solutions to its unique problems, and, true to form, the advent of the Shuttle, and the promise of future launch vehicle developments, demands new and improved solutions for their unique design problems.
This invention involves a class of design problems, thermal insulation, and in particular, thermal insulation for vessels filled with cryogenic liquid propellants. The requirements for thermal insulation of cryogenic liquid propellant tanks are especially rigorous because of the environmental extremes under which it must operate. Liquid hydrogen and liquid oxygen, for example, boil at -252.8.degree. C. and -182.9.degree. C., respectively, a pressure of at one atmosphere. The design of such insulation systems flown with recoverable vehicles such as the Shuttle is made all the more difficult because the insulation system must perform effectively in space and not suffer degradation on the ground before launch or during the atmospheric re-entry process.
NASA requires the thermal insulation for all Shuttle payloads to be exceedingly safe. The safety requirements are especially severe for insulation on tanks of propellants whose surfaces fall below the liquefaction temperature of air. The dangers of this liquefaction of air (comprised primarily of oxygen and nitrogen) are explained below in greater detail.
The design of thermal insulation for Shuttle payloads is further complicated, and perhaps most severely tested, by the dictates of economy. Lower cost is the raison d'etre of the Space Shuttle program and will probably dictate the design of all future launch vehicles. Lower cost is the impetus behind the concept of a reusable launch vehicle and lower cost is the principal reason that the Shuttle is planned to replace the bulk of the conventional launch vehicles, which, in turn, is why the Shuttle is being designed to provide such a broad range of payload services.
In keeping with this philosophy, payloads carried by the Shuttle are encouraged to minimize costs. As an example, one of the payloads envisioned for future Shuttle flight, the Orbital Transfer Vehicle (OTV), exemplifies the low-cost concept and will be discussed in detail. After the Shuttle has attained low earth orbit, the OTV, acting as last stage of a booster rocket, will transfer a spacecraft out of the low orbit to a higher earth orbit, for example, a geosynchronous orbit for weather satellites, or on a trajectory out of the earth's gravitational influence. After releasing the spacecraft, the OTV has the capability to rendezvous either with the Shuttle vehicle from which it left or with another Shuttle vehicle and then be returned by the rendezvous vehicle to earth for reuse.
The thermal insulation for the OTV liquid propellant tanks, which are planned to contain liquid hydrogen and liquid oxygen, must be relatively inexpensive since the Orbiter Transfer Vehicle itself is designed to be low-cost. The low cost of the insulation, however, cannot be a reason for compromising the thermal insulation design requirements mentioned earlier: safety and efficacy on the ground and in space.
Cost has four aspects with regard to Shuttle payloads: weight, space, DDT&E (design, development, testing and evaluation), and the expenses of construction and operation. The first two directly affect Shuttle costs since the Shuttle is weight- and volume-limited. Weight allocated to insulation, for example, cannot be used for additional scientific equipment. Likewise, volume used by insulation cannot be used for other purposes.
DDT&E and construction expenses are one-time expenditures that add to the overall space program costs. Operating expenses, however, will add to the expenses incurred each time the payload is used. Operating expenses include not only what must be spent to use the payloads, but they also include what must be spent to repair the damage to the payload or to its subsystems from earlier flights.
One type of thermal insulation which is used for vehicles in space is multi-layer insulation. Multi-layer insulation is constructed of separate layers of thin, highly reflective sheets that minimize radiation heat transfer.
When multi-layer insulation is used to protect vessels containing cryogenic liquids, however, severe problems can arise during ground operations. The boiling point of hydrogen, for example, is so low that most gases, particularly those comprising the atmosphere, will liquefy on contact with a container of liquid hydrogen. If that container of liquid hydrogen is surrounded only by multi-layer insulation, atmospheric gases can reach the container through the spaces between the layers of insulation. One atmospheric gas, oxygen, liquefies at -182.9.degree. C. Liquid oxygen readily supports combustion of almost any material and is hypergolic with many materials. Nitrogen, the other primary constituent of the atmosphere, liquefies at -209.9.degree. C. The problem resulting from liquefied gases in the layers of insulation is that those liquefied gases tend to accumulate in low points in the insulation and, when exposed to the decreasing atmospheric pressure during launch, will return to the gaseous state and expand in a sufficiently sudden and forceful manner as to damage the insulation severely.
Another problem that arises when using multi-layer insulation on any surface whose temperature falls below the dew point of the surrounding atmosphere is that water will accumulate in the insulation. If the temperature is below 0.degree. C. this water will freeze and if the temperature is above 0.degree. C. it will remain liquid. In either case, during ascent of the launch vehicle, falling atmospheric pressure causes the water (or ice) to boil (or sublimate) suddenly and release, explosively, water vapor that has accumulated over a long period with resultant insulation damage.
There are two techniques for preventing the accumulation of these liquids (and solids) in multi-layer insulation. One technique is vacuum jacketing. In this technique, the insulation is contained within a jacket made up of the tank on the inside and an outer shell capable of withstanding the crushing pressure of the atmosphere when a vacuum is created on the jacket interior. This technique is commonly used for ground based cryogenic tanks, but it is too heavy to be practical for most space applications.
The other technique to protect the multi-layer insulation and to prevent the condensation of ambient gases is to purge the multi-layer insulation with an inert gas whose boiling point is lower than the temperature at the outer surfaces on the cryogenic vessel. In the case of a vessel containing liquid hydrogen, the only readily available purge gas that will work is helium, whose boiling point is -268.9.degree. C. Liquid oxygen tanks may be purged with nitrogen. This multi-layer insulation purging system must operate while the cryogenic vessel is on the ground and perhaps even during the eary phases of launch.